Airbag fabrics

ABSTRACT

A woven fabric comprising spun synthetic polyamide yarn woven in the warp direction and weft direction wherein the polyamide yarn in the woven fabric exhibits a formic acid relative viscosity of at least 60, a halide:copper molar ratio of at least 2.0 and a sodium content of no more than 25 ppm, and wherein the woven fabric exhibits a melt-through resistance at 450° C. of least 2.10 seconds. The invention also provides an airbag made the woven fabric, as well as a method of making the same.

FIELD OF THE INVENTION

The present invention relates to finished woven fabrics comprising spun synthetic polyamide yarn which are suitable as improved airbag fabrics, and a method of making said fabrics.

BACKGROUND TO THE INVENTION

Inflatable airbags are a key component of vehicle safety systems and are installed in virtually every vehicle produced globally. Typically, inflatable airbags are made from woven fabric of nylon or polyester yarns. To meet the requirements for effective inflation, airbag fabric must meet certain tensile strength requirements and have the ability to resist the passage of air, and it is therefore desirable for airbags to have a very low air permeability. As used herein, “airbag” means inflatable passive safety restraints for automobiles and many other forms of transportation, including aviation applications. In recent years, the number of airbags, and the area of coverage for these airbags within various types of vehicular cabins has increased. Multiple air bag configurations in use include airbags for the front seating area, for side impact protection, for rear seat use, for use in headliner area inflatable curtains, and for use in inflatable seat belts or pedestrian airbags. There is a continuing automotive trend towards smaller and lighter vehicles, meaning that less space is available for mandatory safety items such as airbags. It is an object of automobile manufacturers to improve crash impact safety systems in general, and particularly airbag modules, in terms of safety, environmental footprint and cost.

Reduction of airbag module weight per unit area of deployable airbag generally enables total weight reduction without safety compromise. This has become even more important as the number of airbags per vehicle has risen sharply to provide passenger protection at multiple angles. Airbag modules are therefore required to be more efficient in both size and weight.

Reduction of airbag inflator size also enables total weight reduction and increased cost savings. To ensure equal efficacy, the smaller inflators are hotter than historical counterparts.

The trend in the airbag module industry is towards thinner, lighter fabrics used with smaller, hotter inflators. However, this more aggressive design has resulted in experienced events where hot particles and/or hot gases from airbag deployment discharges have penetrated airbag fabrics, injuring vehicle passengers and resulting in the recall of millions of modules. Protection from hot particle penetration and release of hot gases, collectively referred to herein as fabric pinhole failures, becomes increasingly important as the number of airbags per vehicle and their relative proximity to the passenger increases in the effort to improve vehicle safety.

It is an object of this invention to provide airbag fabrics which have a greater resistance to pinhole failure, in order to maintain gains in airbag module weight reduction and improved cost efficiency. Thus, it is a particular object of this invention to provide airbag fabrics which have a greater resistance to pinhole failure and which are also relatively thinner and lighter than conventional airbag fabrics, particularly without detriment to the air permeability or deterioration thereof to unacceptable levels.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a woven fabric comprising spun synthetic polyamide yarn, wherein said fabric is made from polyamide yarn woven in the warp direction and weft direction wherein the polyamide yarn in the woven fabric exhibits a formic acid relative viscosity of at least 60, a halide:copper molar ratio of at least 2.0 and a sodium content of no more than 25 ppm (preferably no more than 20 ppm, preferably no more than 15 ppm, preferably no more than 10 ppm), and wherein the woven fabric exhibits a melt-through resistance at 450° C. of least 2.10 seconds.

The inventors have found that characterization of a fabric's melt-through resistance is able to predict the likelihood of pinhole failures in a full airbag module deployment test.

The inventors have surprisingly found that the formic acid relative viscosity (also referred to herein as “relative viscosity”) of the scoured yarn is a key influence on the melt-through resistance of the finished fabric, and that yarns which exhibit a higher relative viscosity exhibit greater melt-through resistance. Thus, such yarns are able to prevent hot particulate penetration through the airbag during deployment in the new, more effective airbag module designs.

Moreover, the inventors have also surprisingly found that an effective concentration of copper halide in the polymer of the fabric is desirable to maintain the effect provided by the higher relative viscosity, even though neither relative viscosity nor copper halide concentration has a measurable effect on the melting point of the fiber or fabric at the levels described herein.

Furthermore, the inventors have found that, surprisingly, the molar ratio of halide to copper in the finished fabric is desirably maintained above an effective level to maintain the effect which increased polymer relative viscosity has on melt-through resistance.

According to a second aspect of the present invention, there is provided an article, preferably an airbag, made from the woven fabric of the first aspect.

According to a third aspect of the present invention, there is provided a method of making the woven fabric of the first aspect comprising the steps of weaving a spun synthetic polyamide yarn and scouring said yarn before, during or after weaving such that the polyamide yarn in the woven fabric exhibits a formic acid relative viscosity of at least 60, a halide:copper molar ratio of at least 2.0 and a sodium content of no more than 25 ppm (preferably no more than 20 ppm, preferably no more than 15 ppm, preferably no more than 10 ppm).

According to a fourth aspect of the present invention, there is provided the use of a woven fabric according to the first aspect of the invention to improve the resistance to pinhole failure of an airbag made therefrom.

DETAILED DESCRIPTION OF THE INVENTION

The woven fabrics of the present invention are composed of high tenacity spun synthetic polyamide yarns. The yarns are made from fibers which are in the form of continuous filaments. Such filaments are formed by extrusion of molten polymer through spinnerets at high temperatures and pressures, and subsequently quenched in air, coated with spin finish lubricant, drawn between pairs of godets, lightly textured to provide enough entanglement to make a coherent yarn, and then wound up on cardboard tubes, as bobbins.

The spin finish on the filaments facilitates the processing of yarn during its production and is subsequently removed to provide the finished woven fabric. Removal of the spin finish, such as lubricants or oils, as well as anti-static substances, dust, contaminants and the like, from yarn is typically effected by a scouring treatment as conventionally used in the art, for instance by rinsing or soaking with using conventional agents such as water (optionally with defined pH levels), surfactants, detergents, alkalis, sequestrants, emulsifying agents and the like. Such scouring treatments are typically effected during and/or after weaving. In particular, said substances may be removed in the course of the weaving process used to manufacture the woven fabric, for instance a waterjet-weaving process. In this context, the skilled person will appreciate that the term “remove” or “removal” does not necessarily imply complete removal of said substance(s), although complete or substantially complete removal is encompassed by the term.

It will be appreciated that the polyamide yarn in the woven fabric of the present invention is a scoured yarn. As used herein, the term “scoured yarn” refers to a yarn from which spin finish or other lubricants or oils have been removed. The relative viscosity of the yarn is defined herein as the relative viscosity after removal of spin finish from the yarn. Removal of the spin finish may be effected prior to, during or after weaving (preferably during or after weaving). Thus, removal of the spin finish (or “scouring”) may be effected either by a conventional scouring process of the yarn prior to weaving or effected during weaving, and preferably during weaving, for instance in a waterjet-weaving process. Alternatively, removal of the spin finish may be effected after the yarn has been woven into the fabric.

The relative viscosity of the yarn refers to the relative viscosity of the scoured yarn present in the finished woven fabric, which is preferably the same as the relative viscosity of the yarn supplied to the weaving process.

In the woven fabrics of the present invention, at least a majority (and preferably all) of the yarn used in the warp direction of fabric is preferably formed from synthetic fiber made from a single polyamide composition. Similarly, at least a majority (and preferably all) of the yarn used in the weft direction of fabric is preferably formed from synthetic fiber made from a single polyamide composition. In a preferred but non-limiting embodiment, at least a majority (and preferably all) of the yarn used in the warp direction and weft direction of fabric is formed from synthetic fiber formed from a single polyamide composition. Preferably a single polyamide is used in each of the warp and weft directions and preferably the same polyamide is used in both the warp and weft directions.

Suitable polyamide fibers are preferably selected from those formed from nylon 6,6, nylon 6, nylon 6,12, nylon 7, nylon 12, nylon 4,6 or copolymers or blends thereof. In a preferred but non-limiting embodiment, the polyamide is nylon 6,6.

The relative viscosity of the yarn is preferably at least 70, preferably at least 78, preferably at least 85, preferably at least 90, and typically no more than 150, typically no more than 110, typically no more than 100. Preferably, the relative viscosity is in the range of from 60 to 110, preferably 70 to 110, preferably from 85 to 100. The inventors observed no evident relationship between melt-through resistance and the melting point of the polyamide in either the base yarn or the fabric. Thus, the positive correlation observed by the inventors between melt-though resistance and relative viscosity is surprising given that the melting point of the higher-RV polyamide yarn is effectively indistinguishable from the melting point of a corresponding lower-RV polyamide yarn over the relevant relative viscosity range.

Polyamide yarns which exhibit such relative viscosity values may be prepared by means conventional in the art. For instance, relative viscosity may be increased by increasing the degree of polymerization, i.e. the molecular weight, of the polyamide as is known in the art. For instance, the molecular weight and relative viscosity may be increased by a solid state polymerization step, typically conducted under dry nitrogen at elevated temperature (for instance about 180° C.).

In the woven fabrics of the present invention, at least a majority (and preferably all) of the yarn in the warp direction is yarn having a tenacity from 6.8 to 10.1 g/den. Similarly, at least a majority (and preferably all) of the yarn in the weft direction is yarn having a tenacity from 6.8 to 10.1 g/den. In a preferred but non-limiting embodiment, at least a majority (and preferably all) of the yarn in the warp and weft directions is yarn having a tenacity from 6.8 to 10.1 g/den.

The yarn used in the present invention preferably has a linear mass density in the range from about 100 to about 2000 decitex, preferably from about 150 to about 1000 decitex, preferably from about 150 to about 940 decitex, preferably from about 150 to about 750 decitex.

The linear mass density of fiber which constitutes the yarn is preferably in the range from about 1 to about 25 decitex per filament (DPF), or from about 2 to about 12 decitex per filament (DPF).

The woven fabric of the present invention is preferably made from yarn having from 90 to 300 ends/dm, preferably from 160 to 240 ends/dm. Preferably, the woven fabric exhibits a symmetrical construction. Thus, the ends/dm of the warp yarn is preferably the same as the ends/dm of the weft yarn.

The yarn used in the present invention may also comprise various additives used in the production and processing of fibers. Suitable additives include, but are not limited to a thermal stabilizer, antioxidant, photo stabilizer, smoothing agent, antistatic agent, plasticizer, thickening agent, pigment, flame retarder, filler, binder, fixing agent, softening agent or combinations thereof.

Copper compounds have conventionally been added to polyamide yarn, typically either before or during fiber spinning (and preferably added to the polyamide prior to spinning), in order to improve long-term oxidative degradation of the airbag fabric over long periods of storage. The effectiveness of these additives has conventionally been measured by retention of fabric strength (tenacity) over long storage intervals at elevated temperatures. Suitable copper compounds include copper halides (preferably copper bromide and/or copper iodide), copper acetate, copper phosphate, copper salicylate, copper stearate and copper benzoate, as well as copper compounds with xylenediamine, mercaptobenzimidazole or benzimidazole. The copper compound may be in oxidation state I or II. The inventors surprisingly observed that the presence of copper in polyamide positively correlates with improvements in melt-through resistance, despite the fact that the melting points of the polyamide remain unchanged. Even more surprisingly, the inventors observed a relationship between the presence of copper and the higher relative viscosity of the polyamide which together provide further improvements in melt-through resistance. The copper compound is preferably present in the range of 10-500 ppm, preferably 50-150 ppm, preferably 60-120 ppm (calculated as elemental copper) by weight of the polyamide yarn in the finished fabric. Copper bromide and/or copper iodide are particularly preferred.

The inventors have determined that the effectiveness of copper in improving the melt-through resistance of woven fabric correlates with the halide:copper molar ratio in the yarn.

Preferably, the halide:copper molar ratio in the yarn of the woven fabric of the present invention (i.e. the halide:copper molar ratio in the finished fabric) is at least 2.0:1, preferably at least 3.0:1, preferably at least 4.0:1, preferably at least 6.0:1. The inventors have found that such halide-copper molar ratios unexpectedly improve the melt-through resistance. Preferably the halide:copper molar ratio in the finished fabric is no more than 25.0:1, preferably no more than 20.0:1. Thus, preferably the halide:copper ratio is in the range of 2.0:1 to 25.0:1, preferably 3.0:1 to 25.0:1, preferably 4.0:1 to 25.0:1, preferably 6.0:1 to 25.0:1.

The inventors have further determined that the concentration of halide (in particular bromide and/or iodide) in the yarn was not maintained after fabric formation, which was attributed to losses to water in the scouring and rinsing steps before or after weaving, and to any water in the weaving process (for instance waterjet weaving). Exposing fiber or finished fabric to water tends to remove halides, with the rate of loss depending upon a number of factors, including the volume of water, temperature, pH and the identity of the base used in the scouring process. Such yarn and fabric processing typically use scouring agents such as potassium hydroxide or sodium hydroxide. Typically, about 15-50% of the halide content of the yarn is lost during such yarn and fabric processing (which are collectively referred to herein in this context as “scouring processes”), and 80% or more in a harsh scouring process.

Preferably, the halide:copper molar ratio is maintained in the present invention by controlling the harshness of the scouring process in order to minimize halide loss and to ensure a sufficiently high molar ratio in the finished fabric. Scouring treatments to remove spin finishes can be achieved by various means and are well understood in the art, and hence can be modulated accordingly by the skilled person, for instance by controlling one or more of the volume of water, temperature, pH, residence time, and the identity and/or concentration of the base used in the process. Scouring treatments are also sometimes employed to finalize construction or dimensions of the final fabric; for instance, a mild scouring treatment employed for that purpose is typically conducted by rinsing in water at elevated temperatures.

Preferably, the temperature in the scouring process is less than 95° C., preferably less than 75° C.

Preferably, the final pH of a scouring bath is at least 10, preferably at least 11.

Preferably, the residence time in a scouring bath is no more than 5 min/m², preferably no more than 3 min/m², preferably no more than 2 min/m² of fabric.

Preferably, the concentration of sodium ion in the scouring bath or fluid is less than 35 ppm, preferably less than 10 ppm.

In one embodiment, the scouring bath contains potassium hydroxide rather than sodium hydroxide as the base to control pH.

The harshness of the scouring process is suitably determined by measurement of the residual sodium content of the finished fabric, a lower residual sodium content corresponding to a less harsh scouring process. The residual sodium content is no more than 25 ppm, preferably no more than 20 ppm, preferably no more than 15 ppm, preferably no more than 10 ppm (as elemental sodium) by total weight of the finished woven fabric (i.e. after weaving and scouring).

Alternatively or additionally, the halide:copper molar ratio in the yarn of the woven fabric may be controlled by controlling the halide loading in the base fiber or yarn in order to ensure a halide:copper molar ratio of at least 2.0:1 (preferably at least 4.0:1, preferably at least 6.0:1) in the finished woven fabric. The halide loading in the base yarn is defined herein as the halide loading in the fiber or yarn prior to removal of any spin finish from the yarn. Thus, in this embodiment, the halide loading in the base yarn is preferably such that the halide:copper molar ratio in the base yarn is at least 4.0:1, preferably, at least 8.0:1, preferably at least 10.0:1, preferably at least 12.0:1. It will be appreciated that the appropriate halide loading in the base yarn is preferably determined with knowledge of the subsequent scouring process. However, for mild scouring processes (i.e. in which no more than 50% of the halide content is lost), a halide:copper molar ratio in the base yarn of at least 4.0:1, preferably, at least 8.0:1 or at least 10.0:1 or at least 12.0:1 will suffice to achieve the object of the invention. For yarns and fabrics which are intended to be subject to harsher scouring processes in which 80% or more of the halide content is lost, the halide content in the base yarn may be increased accordingly, for instance to at least 15.0:1 or at least 20.0:1, albeit with increasing manufacturing costs. However, depending on scouring conditions, even high levels of halide are subject to nearly quantitative removal, and resultant loss of melt-through resistance.

In order to control the halide loading in the base yarn, a halide salt (other than a copper halide), such as a potassium halide may be added to the yarn in addition to the copper compound, preferably potassium bromide or potassium iodide. Preferably, said halide salt is added to the polyamide prior to spinning. It will be appreciated that in the present invention this halide salt is not a sodium salt.

In a further embodiment, the concentration of sodium in the final fabric is minimised to the desirable levels described herein by controlling the sodium content in the water used in the weaving and/or scouring treatment steps, for instance by a treatment system for the feed water. Thus, In order to minimize the unintentional introduction of sodium ions into the fabric in waterjet weaving, the sodium content of the weaving water may be controlled, for instance by a thin-film reverse-osmosis system capable of rejecting at least 90% of sodium ions in the feed water. Alternatively or additionally, in order to minimize the unintentional introduction of sodium ions into the fabric as a result of the scouring process, the sodium content of water used in the scouring treatment may be controlled, for instance by a thin film reverse osmosis system capable of rejecting at least 90% of sodium ions in the feed water. The water in either the weaving or the scouring step may be recycled. Spin finishes are typically rich in oils and fats, and in order to avoid degrading the performance of the water treatment system by blocking the reverse-osmosis system, it is preferred to utilise an ultrafilter membrane system to screen out any fats or oils prior to removal of the sodium.

The woven fabric of the present invention may be formed from warp and weft yarns using weaving techniques known in the art. Suitable weaving techniques include, but are not limited to a plain weave, twill weave, satin weave, modified weaves of these types, one piece woven (OPVV) weave, or a multi-axial weave. Suitable looms that can be used for weaving include a waterjet loom, airjet loom or rapier loom, and preferably the loom is a waterjet loom. These looms can also be used in conjunction with a jacquard in order to create an OPW structure. The fabrics may be finished according to any methods known in the art, including drying on loom, scouring, can drying and heat setting. Preferably, the woven fabric of the present invention is a waterjet woven fabric which is dried on loom, or dried by a separate process. In waterjet weaving, the dissolution of spin finish in water, and the rubbing of yarns against one another and the heddles and reed of the loom, causes removal of the spin finish lubricant from the yarn.

The woven fabrics of the present invention preferably exhibit a total fabric weight of from 50 to 500 g/m², preferably no more than 300 g/m², preferably no more than 260 g/m², preferably no more than 225 g/m², and preferably at least about 80 g/m², preferably at least about 100 g/m², preferably at least about 150 g/m², and typically at least 170 g/m². In a preferred embodiment, the woven fabrics of the present invention exhibit a total fabric weight of from 150 to 260 g/m², preferably from 170 to 225 g/m².

The woven fabrics of the present invention preferably exhibit a fabric density of no more than 750 kg/m³, preferably no more than 725 kg/m³, typically no more than 700 kg/m³.

The total thickness of the woven fabrics of the present invention is preferably no more than 0.40 mm.

The melt-through resistance of the woven fabrics of the present invention at 450° C., measured as described herein, is preferably at least 2.10 seconds, preferably at least 2.20 seconds, preferably at least 2.30 seconds, preferably at least 2.40 seconds, preferably at least 2.50 seconds, preferably at least 2.60 seconds. The inventors have observed that melt-through resistance increases with increasing fabric weight.

The woven fabric of the present invention preferably exhibits a static air permeability (SAP) of no more than 6.0, preferably no more than 5.0, preferably no more than 4.0, preferably no more than 3.0, preferably no more than 2.0 l/dm²/min when the fabric is unaged.

The woven fabric of the present invention preferably exhibits a dynamic air permeability (DAP) of no more than 700, preferably no more than 600, preferably no more than 500, preferably no more than 400, preferably no more than 300, preferably no more than 200 mm/s when the fabric is unaged.

Preferably, the tear strength of the fabric in both the warp and weft directions is at least 120 N, preferably at least 150 N, preferably at least 170 N when the fabric is unaged.

The woven fabrics of the present invention are preferably uncoated. There are known in the art coated woven fabrics which comprise layers or coatings applied to the surface of the woven fabric for the purpose of reducing air permeability. Such prior art woven fabrics containing additional layers or coatings are referred to herein as “coated woven fabrics”, and take the form of any coating, web, net, laminate or film, which may have been used, for instance, to impart a reduction in air permeability or improvement in thermal resistance. Examples of such coatings include polychloroprene, silicone based coatings, polydimethylenesiloxane, polyurethane and rubber compositions. Examples of such webs, nets and films include polyurethane, polyacrylate, polyamide, polyester, polyolefins, polyolefin elastomers and blends and copolymers thereof. It will be appreciated that the preferred uncoated woven fabrics of the present invention are not “coated woven fabrics” as defined herein. In one embodiment, the woven fabrics of the present invention are further processed by applying said layers or coatings to the surface of the woven fabric to further improve the fabric resistance to pinhole creation by excessively hot gas and/or hot particulates in the process of high energy (hot) inflation.

In a second aspect, the present invention further provides an article made from the woven fabric described herein, wherein the article is selected from an airbag, sailcloth, inflatable slides, temporary shelters, tents, ducts, coverings and printed media, and particularly wherein the article is an airbag. The term “airbags”, as used herein, includes airbag cushions. Airbag cushions are typically formed from multiple panels of fabrics and can be rapidly inflated. Fabric of the present invention can be used in airbags sewn from multiple pieces of fabric or from a one piece woven (OPVV) fabric. One Piece Woven (OPW) fabric can be made from any method known to those skilled in the art.

According to a third aspect of the present invention, there is provided a method of making a woven fabric as described herein comprising the steps of weaving a spun synthetic polyamide yarn and scouring said yarn before, during or after weaving such that the polyamide yarn in the woven fabric exhibits a formic acid relative viscosity of at least 60, a halide:copper molar ratio of at least 2.0 and a sodium content of no more than 25 ppm.

As described hereinabove, the method of making the woven fabric preferably comprises controlling the harshness of the scouring treatment. Alternatively or additionally, the method of making the woven fabric comprises controlling the halide loading in the base yarn from which the woven fabric is made in order to ensure a halide:copper molar ratio of at least 2.0:1 (preferably at least 4.0:1, preferably at least 6.0:1) in the finished woven fabric, preferably wherein the halide loading in the base yarn is such that the halide:copper molar ratio in the base yarn is at least 4.0:1, preferably, at least 8.0:1, preferably at least 10.0:1, preferably at least 12.0:1. Alternatively or additionally, the method comprises controlling the concentration of sodium in the woven fabric by controlling the sodium content in the feed-water used in the weaving and/or scouring treatment steps, for instance by a thin-film reverse-osmosis system capable of rejecting at least 90% of sodium ions in the feed water.

According to a fourth aspect of the present invention, there is provided the use of a woven fabric according to the first aspect of the invention to improve the resistance to pinhole failure of an airbag made therefrom, particularly to the levels of melt-through resistance described herein.

It will be appreciated that the preferences and elements described in respect of the first aspect hereinabove are equally applicable to the second, third and fourth aspects.

The following test methods were used to characterize the woven fabrics disclosed herein.

(i) Formic Acid Relative Viscosity

The relative viscosity (RV) was measured on the fabric according to ASTM D789-19 using a 90% formic acid solution. One 20-gram fabric sample is required for each replicate of this analysis. Prior to RV measurement, each sample was treated to remove any remaining fiber lubricant oil, also known as spin finish. To remove the lubricant, each piece of fabric is soaked in enough methylene chloride to fully cover the sample. The sample is allowed to soak in a covered extraction funnel for twenty minutes with stirring. This procedure is then repeated. Once the second methylene chloride rinse is complete, the fabric is soaked in enough 1:1 methanol:methylene chloride to fully cover the sample. The sample is allowed to soak in a covered extraction funnel for twenty minutes with stirring. This procedure is repeated twice more. Once all five soak steps are complete, remaining solvent is blown out of the fabric sample with clean pressurized air. The fabric is then allowed to air dry completely in an exhaust hood. Once dry, ASTM D789-19 is followed to measure the relative viscosity of the fabric sample.

(ii) Melt-Through Resistance

To measure the melt-through resistance of the woven fabric, a “hot rod” test was used. Each fabric piece is 75 mm wide (warp direction) and 100 cm long (weft direction). Three fabric pieces are required per fabric sample (one per test temperature). Prior to testing, the fabric pieces are conditioned in a controlled atmosphere (20±2° C. and 65±4% RH) for at least 24 hours before testing. This test uses a 12L14 carbon steel cylindrical rod, which is 50 mm in length, 11 mm in diameter and each end being rounded at its edges with a 2 mm radius giving a flat end which is 7 mm in diameter, and weighing 36.5 g, with a specific heat capacity of 502.4 J/(kg° K). The rod is heated to a controlled temperature in a muffle furnace for at least one hour to ensure temperature stabilization before testing. The hot rod is transferred to a delivery tube and brought into contact with a fabric piece which is mounted horizontally below the delivery tube. The fabric pieces are tested at 450° C., 550° C. and 650° C. The first test site must be at least 20 cm from the fabric selvedge. A light-sensor in the delivery tube and a piezo-electric sensor attached to the catch tray positioned underneath the fabric allow a precise measurement of the time required for the rod to penetrate through the fabric once contact is made. The recorded time in the test is the total time (seconds) between the rod breaking the light beam and hitting the catch tray, wherein the total time equals the residence time of the rod on the fabric plus 0.19 seconds (which is the free-fall time it takes for the rod to pass between the light beam and the catch tray with no fabric present). The time required for the rod to melt through the fabric (i.e. the residence time of the rod on the fabric) is then calculated and this time period is defined as the melt-through resistance. Longer melt-through times indicate increased thermal resistance. Each test is repeated 10 times at each temperature to characterize the time required to melt through the fabric sample in seconds.

(iii) Elemental Analysis

Elemental analysis was conducted using neutron activation analysis.

(iv) Fabric Count

Fabric count was assessed using ISO-7211-2.

(v) Fabric Thickness

Thickness testing is conducted on fabric specimens which have been conditioned to standard laboratory conditions of 20±2° C. & 65±4% RH for at least 24 hrs. The specimens are cut from the fabric in such a way that no two specimens possess any common warp or weft yarns. Specimens are not cut within 20 cm of either selvedge or at any creased, obviously damaged or dirty fabric regions. Specimens are suitably cut using a cutter die with a hydraulic press. The thickness of five specimens is measured with an electronic micrometer of testing range 0-25 mm by 0.001 mm (with 6.5 mm diameter jaw faces) and the result recorded. The reported result (in units of mm) is the mean average of five individual specimen results.

(vi) Fabric Weight

Fabric weight was measured according to ISO 3801 (1977) with EASC amendments, and in accordance with EASC instruction 99040180 covering fabric testing (sections 3.05 & 4.01). Weight testing is conducted on samples of fabric which have been conditioned to standard laboratory conditions of 20±2° C. & 65±4% RH for at least 24 hrs. Five square specimens of size 10×10 cm are cut (each orientated on the bias at 45° to the warp direction) from the sample in a diagonal line pattern across the fabric in such a way that no two specimens possess any common warp or weft yarns. Specimens are not cut within 10 cm of either selvedge or at any creased, obviously damaged or dirty fabric regions. Specimens are cut using a 10×10 cm cutter die with a hydraulic press. Once cut, the five specimens are weighed in a 3 decimal place balance in units of grams & the result recorded. Each result is multiplied by 100 to give the fabric weight in g/m2.

The reported fabric weight result is the mean average of five results.

(vii) Static Air Permeability (SAP)

Static Air Permeability SAP was measured according to ISO 9237 (1995) but with the following amendments:

-   (a) The test area is 100 cm². -   (b) The test pressure (partial vacuum) is 500 Pa. -   (c) Each individual test value is corrected for edge leakage. -   (d) Static Air Permeability testing is conducted at six sites on a     test fabric in a sampling pattern across and along the fabric in     order to test 6 separate areas of warp and weft threadlines within     the fabric. -   (e) The reported Static Air Permeability (in units of l/dm²/min) is     the mean average of the six measurements     (viii) Dynamic Air Permeability (DAP)

Dynamic Air Permeability is defined as the average velocity (mm/s) of air or gas in the selected test pressure range of 30-70 kPa, converted to a pressure of 100 kPa (14.2 psi) and a temperature of 20° C. Dynamic Air Permeability is measured according to test standard ASTM D6476-12 but with the following amendments:

-   (a) The limits of the measured pressure range (as set on the test     instrument) are 30-70 kPa -   (b) The start pressure (as set on the test instrument) is adjusted     to achieve a peak pressure -   (c) The test head volume is 400 cm³ unless the specified start     pressure cannot be achieved with this head, in which case an     interchangeable test head of volume 100, 200, 800 or 1600 cm³ is     used, as appropriate for the fabric under test. -   (d) Dynamic Air Permeability testing is conducted at six sites on a     test fabric in a sampling pattern across and along the fabric in     order to test 6 separate areas of warp and weft threadlines within     the fabric. -   (e) The reported Dynamic Air Permeability (in units of mm/second) is     the mean average of the six measurements.

(ix) Tear Force

The tear force (also known as tear strength) of the fabric, expressed in Newtons (N), is determined according to standard ISO 13937-2 (2000) with the amendments as listed below:

-   (a) The fabric specimen size is 150 mm×200 mm (with a 100 mm slit     extending from the midpoint of the narrow end to the center. -   (b) Tear testing is conducted on 5 warp direction & 5 weft direction     specimens cut from each test fabric in a diagonal cross pattern and     avoiding any areas within 200 mm of the fabric selvedges. -   (c) Warp direction tear results are obtained from tested specimens     where the tear is made across the warp (i.e. warp threadlines are     torn) whilst weft direction results are obtained from tested     specimens where the tear is made across the weft (i.e. weft     threadlines are torn). -   (d) Each leg of the specimens is folded in half to be secured in the     Instron clamp grips according to ISO 13937-2 annex D/D.2 -   (e) Evaluation of test results is according to ISO 13937-2 section     10.2 “Calculation using electronic devices”. -   (f) The warp tear force is reported as the mean average of the tear     force results of the five warp direction specimens, whilst the weft     tear force is reported as the mean average of the tear force results     of the five weft direction specimens, in Newtons (N).

(x) Calculations of Fabric Density

The fabric density is calculated by dividing the fabric weight per unit area (g/m²) by the fabric thickness measurement (mm) with a conversion to units of kg/m³.

The present invention is further illustrated by the following examples. It will be appreciated that the examples are for illustrative purposes only and are not intended to limit the invention as described above. Modification of detail may be made thereto without departing from the scope of the invention.

EXAMPLES Example 1

A series of woven fabrics was manufactured with a construction of 209×209 ends/dm, using each using a nylon 6,6 yarn of differing relative viscosities. In each fabric, the warp yarn was the same as the weft yarn. Melt-through resistance of each fabric was tested at 450° C., 550° C. and 650° C. The results, shown in FIG. 1 , demonstrate that melt-through resistance was unexpectedly found to increase with relative viscosity (RV). Thus, it can be seen from FIG. 1 that increasing the relative viscosity from about 70 to about 85 desirably increases the melt-through resistance by 0.2 seconds, which is a significant improvement on the time-scale of airbag deployment. The most desirable melt-through resistance performance was achieved at an RV of at least 78, preferably at least 85.

Example 2

A further series of woven fabrics was manufactured with a construction of 209×209 ends/dm, using each using a nylon 6,6 yarn of differing halide:Cu molar ratios. In each fabric, the warp yarn was the same as the weft yarn, and the yarns in the finished fabric exhibited a relative viscosity of 66 and a residual sodium content of less than 10 ppm. Melt-through resistance of each fabric was tested at 450° C., 550° C. and 650° C. The results, shown in FIG. 2 , demonstrate that melt-through resistance for a fabric with a relatively higher halide:Cu molar ratio was superior to a fabric with a relatively lower halide:Cu molar ratio.

Further investigation of the melt-through resistance at 450° C. of a range of other fabric samples (all with construction 209×209 ends/dm) confirmed that effect, as shown in FIG. 3 . The most desirable melt-through resistance performance was achieved at a halide:Cu molar ratio of at least 4.0:1, preferably at least 6.0:1. Thus, for 60 ppm Cu, the fabric should preferably maintain at least 302 ppm bromine or at least 480 ppm iodine for effective melt-through resistance.

Example 3

A further series of woven fabrics was manufactured from nylon 6,6 yarn, similar to those described above, and using different scouring conditions in order to assess the effect on the halide:Cu molar ratio of (i) residence time in the scouring process, (ii) pH of the scouring bath, and (iii) temperature of the scouring bath.

The inventors observed that halide:Cu molar ratio is inversely related to fabric dwell time in the scouring process, as shown in FIG. 4 . The inventors also observed that halide:Cu molar ratio in the scoured fabric is correlated with the pH of the scouring bath, as shown in FIG. 5 . The inventors also observed that halide losses observed as a result of exposure to alkaline scouring were not mitigated by pH neutralization of the scouring bath composition with weak acids. Surprisingly, weak acids were found to exacerbate halide leaching from the yarn or fabric in such neutralized bath compositions. The inventors further observed that the halide:Cu molar ratio in the fabric is also inversely related to the scouring bath temperature, as shown in FIG. 6 .

Example 4

A further series of woven fabrics were prepared, similar to those described above, in order to investigate the residual sodium content. The inventors observed that fabrics with the same construction (209×209 ends/dm) and relative viscosity (66) but with differing residual sodium content had different melt-through resistance, as shown in FIG. 7 . Thus, the fabric with the low residual sodium content had a higher melt-through resistance. An analysis of a range of woven nylon 6,6 fabrics with the same construction but with differing relative viscosities (65 to 86) showed the same correlation.

Example 5

The inventors also observed that fabrics with a higher relative viscosity (RV) were found to be more sensitive to the impact of residual sodium, i.e. that differing sodium levels had a greater impact on melt-through resistance at higher relative viscosities. High RV (at least 75) fabrics and low RV (less than 75) fabrics with the same construction (209×209 ends/dm) were scoured under various aggressive commercial scouring conditions using various concentrations of sodium. The results are presented in FIG. 8 , which provides further confirmation that fabrics with higher residual sodium levels show poorer melt-through resistance, and demonstrates that this effect is more pronounced for higher RV fabrics.

Example 6

A further series of experiments were conducted in order to investigate the correlation of residual sodium levels with melt-through resistance. Sodium was added to greige woven fabrics by soaking in NaOH or NaCl baths for shorter times and lower temperatures than commercially effective scouring processes. No effect on melt-through resistance was observed, demonstrating that the presence of sodium on the fabric does not itself cause reduced melt-through resistance.

Instead, the residual sodium content is an indicator of the harshness of the scouring process, and that it is exposure to sodium under harsh conditions that results in poor melt-through resistance.

The inventors also found that rinsing harshly scoured fabric in deionized water at various temperatures and soak times resulted in lower fabric sodium content but did not restore melt-through resistance, confirming that the residual sodium content in the finished fabric is indicative but not causative of the reduced melt-through resistance, and indicative of the relative harshness of the scouring process. Thus, the selection of scouring conditions is important to maximize the melt-through resistance of a given polyamide woven fabric. 

1. A woven fabric comprising spun synthetic polyamide yarn woven in the warp direction and weft direction wherein the polyamide yarn in the woven fabric exhibits a formic acid relative viscosity of at least 60, a halide:copper molar ratio of at least 2.0 and a sodium content of no more than 25 ppm, and wherein the woven fabric exhibits a melt-through resistance at 450° C. of least 2.10 seconds.
 2. A woven fabric according to claim 1 wherein said relative viscosity is at least 70, preferably at least 78, preferably at least 85, preferably at least
 90. 3. A woven fabric according to claim 1 wherein the polyamide yarn comprises copper bromide or iodide.
 4. A woven fabric according to claim 1 wherein the molar ratio of halide:copper in said polyamide yarn of the woven fabric is at least 4.0:1, preferably at least 6.0:1.
 5. A woven fabric according to claim 1 wherein said polyamide yarn exhibits a copper content of 60 to 120 ppm calculated as elemental copper by total weight of the polyamide yarn in the woven fabric.
 6. A woven fabric according to claim 1 wherein said polyamide yarn exhibits a sodium content of no more than 20 ppm, preferably no more than 15 ppm, preferably no more 10 ppm.
 7. A woven fabric according to claim 1 wherein the polyamide yarn from which the woven fabric is made exhibits a halide:copper molar ratio of at least 4.0:1, preferably at least 8.0:1, preferably at least 10.0:1, preferably at least 12.0:1.
 8. A woven fabric according to claim 1 wherein the polyamide yarn from which the woven fabric is made further comprises a halide salt other than copper halide or sodium halide, preferably potassium bromide or iodide.
 9. A woven fabric according to claim 1 wherein the yarn in the warp and weft directions is yarn having a tenacity from 6.8 to 10.1 g/den.
 10. A woven fabric according to claim 1 which exhibits a melt through resistance at 450° C. of at least 2.20, preferably at least 2.30, preferably at least 2.40, preferably at least 2.50 seconds.
 11. A woven fabric according to claim 1 which has a total fabric weight of from 50 to 500 g/m², preferably no more than 300 g/m², preferably no more than 260 g/m², preferably no more than 225 g/m², preferably in the range of 170 to 225 g/m², and preferably no more than 220 g/m², preferably no more than 210 g/m².
 12. A woven fabric according to claim 1 wherein said yarn has from 90 to 300 ends/dm, or from 160 to 240 ends/dm.
 13. A woven fabric according to claim 1 wherein said polyamide is nylon-6,6.
 14. A woven fabric according to claim 1 wherein said yarn has a linear mass density in the range of from 150 to 940 decitex, or in the range of 150 to 750 decitex.
 15. A woven fabric according to claim 1 which has a total thickness of no more than 0.40 mm.
 16. A woven fabric according to claim 1 wherein the tear strength of the fabric in both the warp and weft directions is at least 120 N, preferably at least 150 N, preferably at least 170 N when the fabric is unaged.
 17. A woven fabric according to claim 1 preceding claim wherein said fabric exhibits a static air permeability of no more than 6.0, preferably no more than 5.0, preferably no more than 4.0 l/dm²/min, preferably no more than 3.0 l/dm²/min, preferably no more than 2.0 l/dm²/min.
 18. A woven fabric according to claim 1 wherein said fabric exhibits a dynamic air permeability of no more than 700, preferably no more than 600, preferably no more than 500, preferably no more than 400, preferably no more than 300, preferably no more than 200 mm/s.
 19. (canceled)
 20. A woven fabric according to claim 1 wherein the fabric density is no more than 750 kg/m³.
 21. An article, preferably an airbag, made from the woven fabric of claim
 1. 22-27 (canceled) 